1. Field of the Invention
This invention relates to a telemetry system for use in an implantable medical device, such as a pacemaker. More specifically, it relates to a telemetry system for use with a dual-chamber pacemaker for transmitting an indication of when various sensed or paced events have occurred. The telemetered data is received and decoded at a remote receiver where it may be displayed and used for diagnostic purposes.
2. Description of the Prior Art
Cardiac pacemakers which supply electrical stimulation to the heart in the absence of natural occurring heartbeats are well known. Traditionally, such pacemakers were manufactured utilizing discrete analog circuitry. More recently, pacemakers have been designed with digital circuitry of great complexity. This additional capability has been used to expand the operating modalities of the pacemaker to encompass both chambers of the heart and has also been used to add desirable features such as programmability and telemetry capabilities. These modern pacemakers interact with the heart in a complex fashion which complicates follow-up care if traditional pacemaker diagnostic techniques are used.
It has proved desirable to check the pacemaker patient on a routine basis to verify the proper operation of the pacemaker. Traditionally, this task has been accomplished with the aid of an electrocardiogram (ECG) which records the electrical activity detected on the skin surface of the patient. This ECG will display the physiological waveform of the heart as a complex periodic waveform with P, Q, R, S and T portions. This ECG will also display the pacemaker stimulating pulse as a narrow pacemaker artifact on the same ECG trace. By noting the relationship between the pacemaker artifact and various elements of the physiological waveform, the physician can analyze the operating characteristics of the pacemaker to verify its proper and safe performance. However, modern dual-chamber pacemakers have responses which may be difficult to diagnose based solely on the electrocardiogram. Consequently, there is a need to provide additional information to the attending physician to simplify the analysis of pacemaker operation.
One prior art technique which is related to the problem stated above is taught by Dabolt in U.S. Pat. No. 3,662,759. In Dabolt a narrow sub-threshold pulse is applied to the heart via the lead system each time the demand pacemaker escape interval is reset by sensed spontaneous activity. This sub-threshold pulse is insufficient to stimulate the heart; however, its steep rise time generates sufficient radio frequency harmonics to be detected by a conventional radio receiver. In operation a radio "click" is produced each time a naturally occurring R-wave is detected and used by the pacemaker circuitry to reset the escape interval of the pacemaker. The objective of Dabolt is to verify proper sensing by the pacemaker of the naturally occurring heartbeat. Although Dabolt's system provides a convenient method of producing a remote indication of a sensed event with a minimum of equipment, no permanent record is produced by this technique nor is the system applicable to the analysis of more complex dual-chamber pacemakers.